1. Field of the Invention
The present invention relates to light attenuation sensing apparatus.
Many physical parameters and other variables can be measured or sensed using optical transducers which convert the parameter to be measured into an optical signal. For example, known displacement sensors include optical sensors in which a coded track is moved between a source of light and a photo detector so that the displacement, or the position of the track relative to the sensor, can be determined. Such devices require electronic components, for example the photo detector itself, to be located immediately at the displacement sensor. This requires on the one hand the provision of an electrical supply to the sensor, and on the other hand the need to feed from the sensor electronic singals representing the displacement measured.
In some applications, it is desirable to provide transducers for measuring various parameters, including for example displacement, which contain no electronic components and which include no electrical interconnections between the transducer and remote apparatus in which the measured parameter is to be utilised. It has been proposed to employ transducers employing optical techniques, connected to the remote utilisation apparatus by means of optical fibres. Various sensors for measuring different parameters in this way have been proposed in the art.
2. The Prior Art
European Patent Specification No. 0091394 (which is equivalent to U.S. Pat. No. 4,562,348) and European Patent Specification No. 0095673 (which is equivalent to U.S. Pat. No. 4,569,570) disclose optical sensors employing the time delay characteristic of a luminescent material to measure various selected physcial quantities. The sensor is connected by an optical fibre to apparatus for generating light pulses to energise the sensor and for receiving and analysing light signals from the sensor.
United Kingdom Pat. No. 1,081,109 discloses a rod of light conducting material located to collect reflected or transmitted light from a light source which extends across the width of a web of material being checked for flaws. Light collected in the rod is conducted to the ends of the tube for detection by photo-detectors. In order to collect transmitted or specularly reflected light, the rod is formed at least partially of fluorescent material. The disclosed apparatus is capable of detecting holes or other flaws in the material web. However the light conducting rod is used simply to collect light from all points across the width of the web, thereby avoiding the need for a large number of photo-detectors.
German Specification No. 1211421 to Sick is equivalent to the above UK Pat. No. 1,081,109 and there is a further German No. 2009001 also to Sick which is a Patent of Addition to No. 1211421. This German No. 2009001 discloses an improvement in the apparatus of No. 1211421, whereby the illuminating light is amplitude modulated at a set frequency, and the detected light amplitude then filtered, so as to distinguish reflected or transmitted light from background illumination.
United Kingdom Pat. No. 1,493,843 and its equivalent German Pat. No. 2,433,683, also to Sick, disclose a further development of the apparatus for detecting flaws in webs. Two collecting rods are provided, one for detecting scattered light from the web under test and one for detecting specularly reflected light. Also photo-detectors are provided at opposite ends of the rods, with different colour filters, so that the armount of reflected and scattered light at different regions of the spectrum can be measured.
French Specification No. 2545925 to Brunner et al discloses an arrangement for monitoring the edge of a travelling web. Light from a lamp illuminates an elongate detector partially occluded by the edge of the web material. The elongate detector has a strip or tube of fluorescent material mounted in a transparent containing tube. Photo-diodes are mounted all along an edge of the strip to detect fluorescence in the strip caused by impinging light from the lamp. The photo-diodes are connected in parallel, so that the total current drawn from a supply is indicative of the amount of the strip which is illuminated.